1. Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging, in particular to a magnetic resonance imaging method and device achieving water/fat separation.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) is a technology which uses the phenomenon of magnetic resonance to perform imaging. The principles of the phenomenon of magnetic resonance mainly include: in atomic nuclei containing a single proton, such as the hydrogen atomic nuclei which are present throughout the human body, the protons have spin motion and as such resemble small magnets. Moreover, the spin axes of these small magnets have no definite regular pattern, and if an external magnetic field is applied, these small magnets will rearrange according to the magnetic force lines of the external magnetic field; specifically, they will rearrange in two directions, parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is known as the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is known as the negative longitudinal axis. The atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field, so that the spin axes of these atomic nuclei deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance—this is the phenomenon of magnetic resonance. Once the spin axes of the above atomic nuclei have deviated from the positive longitudinal axis or negative longitudinal axis, the atomic nuclei have a transverse magnetization component.
After transmission of the RF pulse has stopped, the excited atomic nuclei emit an echo signal, releasing the absorbed energy gradually in the form of electromagnetic waves, and the phases and energy levels thereof all return to the pre-excitation state. An image can be reconstructed by subjecting the echo signal emitted by the atomic nuclei to further processing, such as spatial encoding.
Since the hydrogen atomic nuclei in fat and the hydrogen atomic nuclei in water inside the human body are in different molecular environments, they have different resonance frequencies when excitation is carried out using the same RF pulses. If signals are collected at different echo times, fat tissue and water display different phases and signal strengths.
Dixon methods are used to create a pure water proton image in MRI. The basic principle thereof is that two kinds of echo signals, in-phase and opposite-phase, of water protons and fat protons are collected separately; these two kinds of signal with different phases are subjected to an operation, each generating a pure water proton image and a pure fat proton image, thereby achieving the objective of fat suppression in the water proton image. There are many forms of Dixon method, including single-point Dixon methods, two-point Dixon methods, three-point Dixon methods and multi-point Dixon methods.
There are many types of k-space data acquisition method that are combined with Dixon methods in the art, for example Cartesian trajectory acquisition and radial or spiral trajectory acquisition, etc. It may be found through research that although existing Cartesian trajectory acquisition methods are simple and save time, they are very sensitive to movement, such as rigid body motion and pulsation. Radial or spiral trajectory acquisition methods, on the other hand, convert motion artifacts to fuzziness in the reconstructed image, involve complex calculation and are extremely time-consuming. Thus neither of the above two method types can eliminate rigid body motion artifacts.
In addition, existing three-point Dixon methods use phase unwrapping techniques to calculate water and fat images, and due to the intrinsic instability of phase unwrapping, the water and fat images calculated may be swapped. That is to say, when an image is theoretically believed to be a water image, the image actually calculated might be a fat image; and when an image is theoretically believed to be a fat image, the image actually calculated might be a water image. Therefore when an object to be imaged is subjected to multi-layer scanning, swapping of the water and fat images calculated may occur in some of the scanning layers, and this leads to errors in the synthesized three-dimensional water image and fat image.